For the most part tank car valves have been operable from the top manhole of the tank. However, this is disadvantageous because if the valve is in some way defective or inoperative, or if the mechanism requires cleaning, an operator must do so from the inside of the car. The lading must be cleaned out of the car for this to take place. Time for the operator to remove the lading and clean the inside of the car is an expensive maintenance operation for the tank car owner or lessee. It is particularly important to be able to replace the entire valve plug operating mechanism while the tank contains lading and the plug portion of the valve remains in place.
Currently, the problem of derailments suggests that it would be preferable for the outlet mechanism to shear off rather than punch or tear a hole in the tank. Therefore it is desired that there be readily shearable planes in the valve which will shear off in the event of impact between the valve and the ground, or upon impact between the valve and articles in its path.
Furthermore, leakage is always a problem and it is desired to provide a valve design which will provide positive sealing to provide assurance against leakage.
One bottom outlet is disclosed in U.S. Pat. No. 1,565,913 (1925). In this patent a valve outlet assembly is riveted to the tank with rivets 16. A transverse operator 31 is utilized to move a valve member 24 into, and out of engagement with cylindrical threaded housing 18. A stop 32 is provided to stop the valve from opening further in the open position.
However, this valve could not be worked on unless the car were cleaned out. The operator would have to work on the valve from inside the car and the operating mechanism could not be replaced with lading in the tank. Furthermore, the necessary shear plates are not found in this valve.
Some ball valves have been provided on tank cars which are operable from the bottom exterior of the tank. See, for example, U.S. Pat. Nos. 3,227,101 (1966); 3,591,131 (1971) and 3,661,355 (1972). However, the operating mechanism for these ball valves could not be replaced with lading in the tank. Furthermore, these ball valves are quite expensive and it is desired to obtain a design which is less expensive than a ball valve.
Angle valves have also been utilized in tank cars. See, for example, pages 3, 5 and 7 of the catalog REGO, RAILROAD TANK CAR VALVES, 4201 West Peterson Avenue, Chicago, Ill. Copyright 1970 Rego Division of Golconda Corp. (copy attached). Angle valves utilizing a handwheel and a vertically extending threaded shaft cooperate with threads on the fixed housing to move the valve member between an open and closed positions. However, even if such an angle valve were reversed and used as an outlet valve (and there is no suggestion to do so) the operating mechanism of the valve could not be replaced with lading in the tank.
This same catalog on page 11 discloses a bottom operated discharge valve which is vertically movable within a strainer 4 and a housing 6 guides the vertical movement. A handle 20B is connected to an operating shaft 15B which is in turn connected to a cam lever arm assembly 16B which raises and lowers the valve. However, the operating mechanism for this valve could not be replaced with lading in the tank, and the shear planes are not provided.
In Melott U.S. Pat. No. 2,002,672 (1935) a valve member 32 is provided with a stop 83 to limit travel of the valve in open position.
In U.S. Pat. No. 2,072,271 (1937) a vertically movable safety valve utilizes guide members 32 to maintain basically vertical movement of the valve head B and prevent the same from being cocked during the vertical movement. However, this is a safety valve movable between open and closed positions against the bias of a spring by pressure from a pressure chamber. It has no operator in sense of an operator for a discharge valve, and thus this valve is not adapted to be used as a bottom discharge outlet.